They tick down as normal due to both turns going by and being attacked. Timeless simply adds to their duration. The unit status screen shows a max of 9 rounds duration for an effect but it can definitely be much, much longer than that.
Last Hero would be hilarious if it went off after 100 turns of soaking up damage
. It's not really needed though since you can already maintain Stone Skin and Divine Armor. Using Avenging Angel is just for showing off though, attack buffs that can easily be pushed to last forever and not require constant Timeless to maintain are better.